System and method of carbon capture and sequestration

ABSTRACT

Systems and methods of capturing and sequestering carbon dioxide, comprising mixing a substantially non-aqueous solvent and an alkali such that the solvent and alkali form a solvent suspension, mixing water and a flue gas containing carbon dioxide with the solvent suspension such that a reaction occurs, the reaction resulting in the formation of a carbonate, water and heat.

CROSS-REFERENCE TO RELATED APPLICATION

This is a reissue application of and claims priority to U.S. Pat. No.7,947,240, issued May 24, 2011.

FIELD OF THE INVENTION

The present invention relates to carbon capture and sequestrationsystems and methods.

BACKGROUND

The capture and sequestration of carbon dioxide (CO₂) emissions needs tobe significantly improved if the climate change consequences of suchemissions are to be controlled or curtailed. The CO₂ produced fromcombustion and industrial processes, specifically power plant flue gas,is perhaps the largest single greenhouse gas emission. Most existingcarbon capture and sequestration methods take a two-step approach.First, a method is sought for separating CO₂ from the flue gas or othergaseous emission source. These may include capture of the CO₂ in liquidsolvents, solid zeolyte or various membranes. However, the capture medianeed to be regenerated without releasing the CO₂ into the atmosphere,and this is difficult to achieve in standard physical separationprocesses.

The second step is sequestering the CO₂ gas or liquid by inserting itinto underground geological formations or in deep ocean layers. However,very specific geological configurations are required for disposal of theCO₂, and these are not commonly available at CO₂ emission sites. Thus,transportation adds substantial cost and difficulty. In addition, it isnot known whether CO₂ can be permanently sequestered underground. Thetwo-step approach also is not economical because often CO₂ representsonly a small percentage of a large volume of flue gas, and treating alarge flow stream to recover a small portion of it as CO₂ is wastefuland expensive.

Another approach to CO₂ capture and sequestration involves mining,crushing and transporting rocks to the emission site, where the crushedrock is used to absorb CO₂. But this requires a good deal of heat andpressure. The energy input and environmental costs of mining the rockand transporting it to and from the CO₂ source, as well as the energycosts of having the crushed rock accept and absorb the CO₂, are veryhigh.

Other ways to capture CO₂ include chemical absorption using liquids suchas amines or aqueous solutions of bases, physical absorption in anappropriate solution and membrane separation. All of these methods havethe problem that the absorption media need to be regenerated withoutlosing CO₂. Other capture methods such as physical adsorption andcryogenic separation require significant amounts of energy in the formof heat or pressure.

Some CO₂ capture methods react CO₂ (or carbonic acid formed from waterand CO₂) with an aqueous solution of an alkali to form a carbonate.However, a significant drawback of that approach is that the carbonateexits the process in solution with water, requiring further, energyintensive treatment to separate the solids and the water, or it resultsin a large volume, heavy, wet, cement-like paste that requires energyintensive drying and mechanical systems to control the size,configuration and weight of the resulting dried product.

Although some are examining techniques for capturing and sequesteringCO₂ from ambient air, they are not suitable for CO₂ emissions from powerplants because of the substantial difference in CO₂ concentrationbetween ambient air and flue gas. Ambient air generally contains betweenabout 0.03% and 0.04% CO₂, whereas flue gas contains 3.0% or higherconcentrations of CO₂. Removing very small quantities of CO₂ from thevery large quantities of ambient air is not as viable and as productiveas the capture and sequestration of large amounts of CO₂ from streams,such as flue gas, where the CO₂ is more concentrated.

Therefore, there exists a need for a commercially viable carbon captureand sequestration process that works at industrial scales and iscomplete and permanent. Specifically, there is a need for a carboncapture system that does not use capture media that require complex andenergy-intensive regeneration, and does not yield a heavy, wet endproduct that requires energy intensive drying and other post-captureprocessing. There is a further need for a carbon capture andsequestration process that permanently sequesters CO₂ at the site of CO₂emission. In summary, a need exists for a carbon capture andsequestration system that is cost effective and not energy intensive andresults in permanent sequestration of CO₂.

SUMMARY OF THE INVENTION

The present invention, in its many embodiments, alleviates to a greatextent the disadvantages of known carbon capture and sequestrationmethods by providing a chemical process by which carbon dioxide in theform of carbonic acid is reacted with an alkali to form water and a dry,easily-removable carbonate that precipitates out of solution. Carbondioxide sequestration is achieved by the above-ground disposal of theresulting carbonate. This process allows for industrial scale CO₂capture and sequestration at relatively low costs. Embodiments of thepresent invention also provide permanent, on-site CO₂ capture andsequestration requiring relatively low energy consumption.

In an embodiment of the present invention, known as Vandor's CarbonCapture and Sequestration Cycle (VCCS), a method of capturing orsequestering carbon dioxide is provided in which a substantiallynon-aqueous solvent is mixed with an alkali such that the solvent andalkali form a solvent suspension. This mixing step may be performed inany suitable mixing vessel. The substantially non-aqueous solventpreferably is an alcohol, and is methanol in a most preferredembodiment. As such, the alkali reacts with the methanol to formmethoxide, which may also include solvated metal hydroxide. Water and aflue gas containing carbon dioxide are mixed with the solvent suspensionsuch that a reaction occurs, the reaction resulting in the formation ofa carbonate, water and heat. The terms “solvent” and “non-aqueoussolvent” will be used interchangeably herein to mean any substantiallynon-aqueous solvent that will tolerate some significant amount of alkalito be dissolved in it, and will force the precipitation of any salt thatis produced in the classic acid+base reaction. The non-aqueous solventcontains less than 50% water, and most preferably less than 10% water.

The gas is preferably flue gas from a power plant, but may be any typeof exhaust gas containing CO₂ from any industrial process. The gas willcontain nitrogen (N₂) as well. The term “flue gas” will be used hereinto mean any exhaust gas stream that contains carbon dioxide and eithernitrogen or air, the exhaust gas being from a power generation plant'sflue, including coal-fired, natural-gas-fired, oil-fired, and landfillgas (LFG)-fired or anaerobic digester (ADG)-fired power plants or fromany industrial process including, but not limited to, cement making inkilns, glass, steel, rubber, paper, or other materials manufacturing,the production of ethanol, and from any combination of flue gas andprocess gas.

In one embodiment, ash is introduced into the solvent, and the alkali isa constituent of the ash. As used herein, the term “ash” will be used tomean fly ash, bottom ash and all types of alkali-containing ash from anysource including from coal burning, wood burning and other bio-massburning.

The chemical process of carbon capture and sequestration comprisesmixing the water and the flue gas containing carbon dioxide with thealkali suspended in the solvent, preferably methoxide, so reactionsoccur that result substantially in the formation of a solid carbonate,water and heat. Small amounts of carbonic acid also are formed in thereactions, and the carbonic acid quickly reacts with the alkali. Thesereactions may be performed in any suitable reaction vessel. In apreferred embodiment, the carbonate precipitates out of solution and isremoved from the vessel. Removal of the precipitated carbonate ispreferably performed mechanically, using an auger or another suitablemechanical device that allows for the removal of solids without anyliquids leaving the vessel at the same location. Any methanol thatremains with the carbonate evaporates upon the addition of modestamounts of low-grade heat.

The water resulting from the reactions in the reaction vessel forms asolution with the solvent, and the method further comprises removing thesolution of water and solvent and separating the water from the solvent.After the water and solvent are separated, the separated solvent isre-mixed with the alkali such that the solvent and alkali again form asolvent suspension that can be used for further carbon capture. Theseparated water is returned to the solvent suspension in the reactionvessel where it joins the flue gas and the methoxide to continue thereaction. In a preferred embodiment, the water is separated from thesolvent by chilling the solution of water and solvent in a cryogenicdrying vessel. When the solution is chilled, the water fallssubstantially to the bottom of the cryogenic drying vessel, and thesolvent rises substantially to the top of the cryogenic drying vessel.In some embodiments, some carbonate will travel with the solution ofwater and solvent and precipitate out of the solution in the cryogenicdrying vessel. A filter may be used to trap larger solids in thereaction vessel, keeping those larger solids from traveling on to thecryogenic drying vessel.

The remaining water may be separated from the solvent using a hotdistillation vessel by applying heat to the solution of water andsolvent to at least partially vaporize the solvent. A partial vacuum maybe used to draw off vaporous solvent from the distillation apparatus,and the vaporous solvent is condensed to a liquid by cooling to be madesuitable for re-use in the carbon capture and sequestration reactions.

Embodiments of the present invention include methods of using nitrogenfrom the flue gas to provide cooling for the carbon capture andsequestration process. The method may include liquefying the nitrogenand recovering refrigeration from the liquefied nitrogen. The recoveredrefrigeration from the nitrogen is then used to cool the solvent andprovide cooling for the solvent regeneration steps. This use of nitrogenfor cooling increases the energy efficiency of embodiments of theinvention.

In a preferred embodiment, the flue gas further contains nitrogen andthe nitrogen is used in three ways. A first portion of the nitrogen isused for refrigeration during the solvent regeneration process, a secondportion is used to enhance the power output of a power plant, and athird portion is sold to off-site customers. All of the nitrogen isfirst compressed. For the portion used for refrigeration, a refrigerantsource provides refrigerant to a heat exchanger, and the nitrogen ischilled in the heat exchanger such that it is substantially liquefied.Refrigeration may be recovered from the substantially liquefied nitrogenafter it is pumped to pressure and sent to the power cycle to enhancethe power output of the power plant that is the source of the flue gas.The recovered refrigeration is used to provide cooling for the cryogenicsolvent removal process, discussed below, that separates the water fromthe solvent.

A second portion of the nitrogen may be used to enhance the power outputof a power plant. In a preferred embodiment, a first portion of thissubstantially liquefied nitrogen is compressed and heated. The heatedcompressed nitrogen is directed to a steam cycle of a power plant toenhance the power output of the power plant. A second portion of thissubstantially liquefied nitrogen may be stored in a storage apparatus.The second portion of the substantially liquefied nitrogen ispressurized by pumping it to pressure. It is then vaporized and directedthrough a hot gas expander to enhance the power output of the powerplant. A third portion of this liquefied nitrogen is sold to off-sitecustomers for a variety of uses, including as a refrigerant and as afluid to enhance oil and gas well recovery. In a preferred embodimentthe liquefied nitrogen is further refined by removing liquid argon,which is approximately 0.9% of the volume of the recovered nitrogenstream, and which is a high-value product that may also be sold in themarketplace.

Embodiments of the present invention include carbon capture andsequestration systems which comprise a carbon capture assembly and asolvent regeneration assembly. The carbon capture assembly comprises amixing vessel and at least one reaction vessel, and may further includea solvent condenser fluidly connected to the reaction vessel. In themixing vessel, an alkali is mixed with a substantially non-aqueoussolvent to form a suspension. In one embodiment, ash is introduced intothe solvent, and the alkali is a constituent of the ash. The non-aqueoussolvent preferably is an alcohol, and is methanol in a most preferredembodiment. As such, the alkali reacts with the methanol in the reactionvessel to form methoxide and possibly some metal hydroxide. Minorquantities of dimethyl-carbonate (DMC) may also form, but will quicklydecompose due the alkaline conditions.

The reaction vessel is fluidly connected to the mixing vessel so itreceives the suspension of alkali and a substantially non-aqueoussolvent from the mixing vessel through a first input. The reactionvessel also receives flue gas containing heat and carbon dioxide througha second input and water through a third input such that carbonic acid,carbonate, water and heat are formed in the reaction vessel. Morespecifically, the carbon dioxide and water and any small amounts ofcarbonic acid that result from the reactions in the reaction vesselreact with the alkali in the vessel, resulting in the formation of acarbonate, water and heat. The flue gas will contain nitrogen as well.In some embodiments, the carbon capture assembly further comprises asolvent condenser fluidly connected to the reaction vessel, whererefrigeration is used to condense the solvent portion of the exitingstream, which consists of mostly nitrogen.

The solvent regeneration assembly is fluidly connected to the reactionvessel and comprises at least one heat exchanger, a cryogenic dryingvessel fluidly connected to the heat exchanger, and a hot distillationvessel fluidly connected to the cryogenic drying vessel. The solventregeneration assembly preferably has a plurality of heat exchangers toperform several intermediate heat recovery steps to warm the mostlywater stream that arrives at the hot distillation vessel and to cool themethanol vapor that leaves the hot distillation vessel.

The carbonate formed in the reaction precipitates out of solution and isremoved from the reaction vessel. The carbon capture assembly mayfurther comprise an auger or other suitable device to remove theprecipitated carbonate from the reaction vessel. The water resultingfrom the reactions forms a solution with the solvent in the reactionvessel, and this solution of water and solvent is removed from thereaction vessel and directed to the solvent regeneration assembly. Thewater is separated from the solvent by the solvent regenerationassembly, and the separated solvent is returned to the mixing vesselwhere it is re-mixed with the alkali to form a solvent suspension. Also,the separated water is returned to the reaction vessel to continue thereactions.

In some embodiments, a small portion of the carbonate (e.g., less than10% by volume) will stay in the solvent and travel with the solventsuspension through the solvent regeneration assembly. When the selectedalkali is CaO, the solution of water and solvent is free of anycarbonates. When the selected alkali is KH, some carbonate will form asolution with the water+solvent. That small portion of carbonate willfall out of the solvent suspension with the water that is separated fromit. First, the separation process uses the cryogenic drying vessel inwhich the solution of water and solvent is chilled so the water fallssubstantially to the bottom of the cryogenic drying vessel, and thesolvent rises substantially to the top of the cryogenic drying vessel.Part (or in a more energy-intensive option, all) of this separationprocess uses the hot distillation vessel, where heat is applied to thesolution of water and solvent, a partial vacuum draws off vaporoussolvent from the hot distillation vessel, and the vaporous solvent iscondensed.

Some embodiments may include a nitrogen liquefaction assembly whichsubstantially liquefies nitrogen contained in the flue gas and recoversrefrigeration from the substantially liquefied nitrogen. The recoveredrefrigeration from the nitrogen may be used to cool the solvent and toprovide cooling for the solvent regeneration assembly. That portion ofthe liquid nitrogen is sent to the regeneration assembly under pressure,having been pumped to pressure by a cryogenic pump. The solventregeneration assembly heats a first portion of the substantiallyliquefied nitrogen and directs the heated nitrogen to a steam cycle of apower plant to enhance the power output of the power plant. A storageapparatus stores a second portion of the substantially liquefiednitrogen, releases the second portion of the substantially liquefiednitrogen, and directs it to a hot gas expander to enhance the poweroutput of a power plant.

Embodiments of the present invention include methods for separatingchemical constituents of flue gas (containing CO₂, a relatively largeportion of N₂, and a much smaller portion of argon) comprising mixing asubstantially non-aqueous solvent and an alkali such that the solventand alkali form a solvent suspension. Water and a flue gas containingcarbon dioxide and nitrogen are introduced to the solvent suspension.The alkali in the solvent suspension is contacted with the water and thecarbon dioxide in the flue gas such that a series of fast-paced chemicalreactions occur. The reactions result in the formation of a carbonate,water and heat, with the unreacted mostly-nitrogen portion leaving thereaction vessel as a gas, and carrying with it small quantities ofvaporized solvent.

That mostly-nitrogen stream is chilled in a solvent condenser so as toliquefy that small solvent portion, which is returned to themethanol+alkali mixing vessel. The remaining mostly-nitrogen gas streamis liquefied by compressing and chilling the nitrogen. In a preferredembodiment, the refrigeration content of the substantially liquefiednitrogen is recovered and used to provide cooling for separating thewater from the solvent. The nitrogen portion used for cooling is firstcompressed by pumping it to pressure using a cryogenic liquid pump andthen heated by recovered heat in the solvent regeneration assembly. Thatnitrogen is then directed to a steam cycle of a power plant, or to agenerator-loaded hot gas expander to enhance the power output of thepower plant. A second portion of the substantially liquefied nitrogen isstored and then may be vaporized and directed through a hot gas expanderto enhance the power output of a power plant. A third portion of thesubstantially liquefied nitrogen is sold to off-site customers.

Accordingly, it is seen that a chemical process for securely and costeffectively capturing and sequestering carbon dioxide on site at a largescale is provided in which carbon dioxide in the form of carbonic acidreacts with an alkali in a solution to form a carbonate, water and heat.These and other features of the present invention will be appreciatedfrom review of the following detailed description of the invention,along with the accompanying figures in which like reference numbersrefer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects of the invention will be apparent uponconsideration of the following detailed description, taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a process diagram of an embodiment of a carbon capture andsequestration system in accordance with the present invention;

FIG. 2 is a process diagram of an embodiment of a solvent regenerationassembly in accordance with the present invention;

FIG. 3 is a process diagram of an embodiment of a carbon capture andsequestration system in accordance with the present invention integratedwith a power plant; and

FIG. 4 is a process diagram of an embodiment of a nitrogen liquefactionassembly in accordance with the present invention.

DETAILED DESCRIPTION

In the following paragraphs, embodiments of the present invention willbe described in detail by way of example with reference to theaccompanying drawings, which are not drawn to scale, and the illustratedcomponents are not necessarily drawn proportionately to one another.Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than as limitations on the presentinvention. As used herein, the “present invention” refers to any one ofthe embodiments of the invention described herein, and any equivalents.Furthermore, reference to various aspects of the invention throughoutthis document does not mean that all claimed embodiments or methods mustinclude the referenced aspects. Reference to temperature, pressure,density and other parameters should be considered as representative andillustrative of the capabilities of embodiments of the invention, andembodiments can operate with a wide variety of such parameters. Itshould be noted that the figures do not show every piece of equipment,nor the pressures, temperatures and flow rates of the various streams.

The examples of gas, liquid, and solid products produced by variousembodiments of the present invention are not intended to becomprehensive. Some minor products of embodiments of the invention,including those that form temporarily and then dissolve, will not bediscussed in great detail below but are understood to be included withinthe scope of the invention. Not all points of heat generation will bementioned below, but it is understood that all worthwhile heat producedin embodiments of the invention will have the potential for heatrecovery and potential use, thus reducing the total energy inputrequired by the process.

FIG. 1 shows two major subsystems of an embodiment of the presentinvention, a carbon capture assembly 100, and a solvent regenerationassembly 200. Carbon capture assembly 100 includes reaction vessel 101and mixing vessel 102 and preferably includes solvent condenser 103. Thesolvent regeneration assembly 200 will be described in detail herein inconnection with FIG. 2. The system shown can be used with any powerplant and with any type of exhaust gas, and is particularly well-suitedfor capturing and sequestering carbon dioxide from flue gas from acoal-fired power plant. Flue gas from engines, such as at LFG sites,produce exhaust gas at close to 900° F. While most such engine-drivesystems do not have heat recovery attachments, the low-grade heatcontent of the flue gas is a significant energy source for embodimentsof the present systems and methods.

The chemical process of carbon capture and sequestration comprisescontacting the CO₂+water and some temporarily formed (small quantities)of carbonic acid 14 with the alkali 2 that is suspended in methoxide 5so there is a reaction that results in the formation of precipitatingcarbonate 6, water-methanol solution 10 and heat. To begin with,CO₂-laden flue gas 1 and water 4 are introduced into the methoxide 5,both streams entering reaction vessel 101 separately at the same time.That separation allows full control over the flow rate of both streamsand allows the water stream 4 to be adjusted in response to any minoramounts of water vapor contained in the flue gas. Reaction vessel 101receives the methoxide suspension 5, which consists of alkali 2 and asubstantially non-aqueous solvent 12, from the mixing vessel 102 througha first input 113, which is preferably an input valve. Reaction vessel101 receives flue gas 1 through a second input 111 and water through athird input 112, both preferably input valves. The reactions between theCO₂+water (and small amounts of temporary carbonic acid 14) and thealkali 2 contained in the methoxide 5 occur rapidly (sometimes in lessthan a second), fully converting the gaseous CO₂ into solid carbonatesand byproducts of water and heat.

In a preferred embodiment, the carbonate 6 precipitates out of solutionand is removed from reaction vessel 101 mechanically, using an auger 104or any other device or system suitable for mechanically removingcarbonate precipitates. In some embodiments, up to approximately 10% ofthe volume of the water-methanol solution 10 remaining in reactionvessel 101 will contain suspended carbonate, which will not fall to thebottom of the reaction vessel but will fall out of solution during themethanol regeneration process. The water resulting from the acid+basereactions forms a solution with the solvent. That water-solvent solution10 is removed through a filter 114, which prevents larger solids fromleaving the reaction vessel, and which will fall to the bottom of thevessel, where they will be mechanically removed. The method furthercomprises removing water-solvent solution 10 from reaction vessel 101and separating the water from the solvent. In those embodiments thatcarry carbonates in the water-solvent solution 10, the carbonates willseparate out with the water. This solution 10 of water and methanol iswithdrawn near the top of reaction vessel 101 at a warm temperature thatreflects the optimum temperature of the reactions, which will minimizethe time required for the reactions.

As a preliminary step, an alkali 2 is mixed with a solvent 12 in mixingvessel 102, to form a suspension 5. Any of a number of alkalis known inthe art can be selected for neutralizing the CO₂ in flue gas, producingtheir respective carbonates. The alkali may be a strong or a weak base,and may include such common bases as sodium hydroxide (NaOH) orpotassium hydroxide (KOH) in powdered form, or hydrides such asmagnesium-, potassium- or sodium hydride (MgH, KH, NaH), or anhydrousammonia, or calcium oxide (CaO) found in the fly ash (and bottom ash)that is another byproduct of coal-fired or biomass power plants andboilers, or any other suitable alkali, natural or synthetic that willreact with the CO₂.

One advantage of embodiments of the present invention is that it can beused to perform carbon capture and sequestration at large industrialscales. Employing the systems and methods described herein at facilitiesof all sizes allows use of multiple alkalis, resulting in theirrespective carbonates. An illustrative list, followed by the chemicalsymbol of each alkali and the carbonate produced when reacted with CO₂and the chemical symbol of each carbonate, is provided here:

Ammonia (anhydrous), NH₃→Ammonium carbonate, (NH₄)₂CO₃

Lithium Hydride, LiH→Lithium carbonate, Li₂CO₃

Lithium Hydroxide, LiOH→Lithium Carbonate, Li₂CO₃

Magnesium Hydride, MgH₂→Magnesium Carbonate, MgCO₃

Magnesium Hydroxide, Mg(OH)₂→Magnesium Carbonate, MgCO₃

Potassium Hydride, KH→Potassium Carbonate, K₂CO₃

Potassium Hydroxide, KOH→Potassium Carbonate, K₂CO₃

Sodium Hydride, NaH→Sodium Carbonate, Na₂CO₃

Sodium Hydroxide, NaOH→Sodium Carbonate, Na₂CO₃

One embodiment uses potassium hydride (KH), possibly in combination withother alkalis. MgH₂ and ash could be used in combination with the KH toincrease the CO₂ capture rate. The hydrides of potassium, sodium,magnesium, (KH, NaH, and MgH, respectively) are less expensive thantheir hydroxide counterparts (KOH, NaOH, Mg[OH]₂), and yield a largeramount of carbonate per unit of hydride than the hydroxides, making thehydrides more economical. Such combinations of alkalis would requiremultiple mixing vessels and multiple reaction vessels. Some hydrogen mayalso form as a by-product of using certain hydrides. For example, about930 L of hydrogen will result from NaH and about 560 L of hydrogen willresult from KH for every two pounds of hydride dissolved in methanol.Such an H₂ stream would not be vented, but would be used as fuel in oneof several possible locations in embodiments of the invention. Forexample, the H₂ stream can be sent directly to the combustion chamber ofthe power plant, or it can be burned in a supplemental heater thatprovides additional heat to the N₂ stream that is used for enhancedpower output. The selection of alkalis and the resultant carbonates willdepend on the markets for those carbonates and the relative costs of thealkalis when compared to the value of the carbonates.

A preferred embodiment uses the alkali present in fly ash, the finepowder recovered from flue gas at coal-fired and biomass power plants orcoal-fired and biomass boilers, prior to the release of the flue gas tothe atmosphere. Similarly, bottom ash, resulting from the remains of thecoal or biomass that does not travel up the flue, is a product for whichuses are sought, but which is still a significant waste stream. Thefollowing discussion on ash covers both fly ash and bottom ash, whichhave similar chemical components, and all other alkaline ash from anysource.

Much of the ash produced at coal-fired power plants does not have a use.Most of it is transported to landfills for disposal, or for otherlow-value applications. Ash from lignite, a widely-used type coal,contains 15-45% SiO₂ (sand), 20-25% AlO₃ (aluminum oxide), 4-15% Fe₂O₃(iron oxide) and 15-40% CaO (calcium oxide), with up to 5% unburnedcarbon. Sub-bituminous coal will produce fly ash with lesser proportionsof CaO (5-30%), which can also be used as an alkali source, butrequiring larger amounts of ash to produce the same results. The removalof the iron oxide by magnetic means, preferably when the ash issuspended in methanol, will serve to concentrate the amount of CaO inthe methoxide, yielding another profitable byproduct (iron oxide) andreducing the weight and transport costs of the final carbonate-ladensolid product stream by the removal of the relatively heavy iron. TheCaO contained in fly ash is the same alkali that one can purchase aslime, but in this context is a byproduct of the burning of coal thatcontained calcium carbonate. Thus, the CaO is obtained from the ash withno additional CO₂ emissions beyond what the power plant normally emits.By contrast, buying manufactured CaO would increase the carbon footprintof this process because manufacturing CaO results in large CO₂emissions.

One embodiment of the carbon capture and sequestration method hosts theash and the CO₂-containing flue gas 1 in methanol 12, substantiallylimiting the amount of water in reaction vessel 101. This allows thereaction to yield a dryer and more controllable (as to size andconfiguration) end product. In this preferred embodiment, the endproduct will be uniformly sized granules, requiring little or nopost-dryer crushing, yielding a suitable agricultural lime substitute,while minimizing the amount of input energy required by the process.

The glass-like ash may benefit from a rapid cooling process that cracksthe microscopic ash particles, thus facilitating the reaction of thealkali in the ash with the CO₂ and water delivered to the reactionvessel by streams 1 and 4. That rapid cooling preferably includes firstwarming the ash and then rapidly cooling it in deeply chilled methanol,thus cracking each glass-like bead of microscopic ash. If the reactionsoccur in warm methanol (as is likely), then the quenching of the ashstream can occur first in one vessel, followed by the mixing of themethanol plus ash solution with warmed methanol in a separate reactionvessel. The heat needed to warm the ash before the rapid cooling may bedelivered from one of the many heat recovery points in the process.

It is preferred that the acid+base reaction occur in a host liquidhaving the alkali, or base, in solution, and allow for easy contactbetween that base and the CO₂+water (plus small amounts of temporarycarbonic acid) that is formed when CO₂ and water are introduced toalkaline-laden solvent. Therefore, preferred embodiments use asubstantially non-aqueous solvent to host the reaction. This isaccomplished by withdrawing from the top of reaction vessel 101 thewater-methanol solution 10, at the same rate as the reaction produceswater, and replacing the water-methanol solution 10 with an equivalentvolume of rich (i.e., substantially water-free) methoxide 5. The amountof water inflow to the reaction vessel is dependent on the water contentof the flue gas and the quantity of water that might remain in solutionin the methanol from prior inflow of flue gas.

In addition, the water that is a product of the acid+base reaction needsto be withdrawn from reaction vessel 101 at a sufficient rate so as toprevent the methoxide 5 from hydrolyzing. The mostly dry flue gas 1 isbubbled through the methoxide 5, along with an appropriate amount ofwater (stream 4), allowing the CO₂ to react with the alkali andtemporarily form small quantities of carbonic acid 14, which also reactswith alkali 2 that is held in solution 5 by the solvent 12. It ispreferred that the flue gas 1 enter reaction vessel 101 at enoughpressure, e.g., approximately 16.5 psia, so that the flue gas 1 can risethrough the host methoxide 5 and allow the unreacted portion of the fluegas (mostly N₂) to leave reaction vessel 101, as a mostly N₂ andvaporized methanol stream 8, which is recovered by condensation insolvent condenser 103. Accounting for pressure drop along thepre-cooling route of the flue gas, the present invention seeks toreceive the flue gas at approximately 17 psia.

In a preferred embodiment, the non-aqueous solvent is an alcohol andmost preferably, methanol. However, any other suitable non-aqueoussolvent that will tolerate some significant amount of alkali to bedissolved in it, and will force the precipitation of any salt that isproduced in the classic acid+base reaction may be used. Ethanol is asomewhat costlier alternative, which may be selected if, for example,the process is used to capture and sequester CO₂ produced at an ethanolplant. In that context, the ethanol will be available at the equivalentof a wholesale price, and make-up ethanol will not require any shipping.The purpose of the solvent is to allow the acid+base reactions to occurwithin a substantially dry liquid, thus avoiding the formation of saltwater or carbonates suspended in water, and avoiding an end product witha high percentage of water that must be driven off.

The alkali 2 mixes with the methanol solvent 12 to form methoxide 5, asolution of methanol and any appropriate hydride or hydroxide base wherethe base is in suspension. The following is one example of a genericchemical equation for the mixing of an alkali (KH, or potassium hydride)with methanol: 2KH+MeOH yields 2MeOK+H₂. The methoxide may berefrigerated to recover and counter-act the heat of reaction that willoccur when some alkalis are introduced into methanol. The choice of howcold the methoxide should be will depend on which alkali is selected andwhich carbonate will be the end product of the reaction, and by themethods selected for controlling the temperature of reaction vessel 101,and thus limiting the boil off of methanol from the reaction vessel.

Mixing the alkali 2 with ambient temperature methanol 12 in mixingvessel 102 creates heat as the two compounds interact, and will producean ionic solution of methoxide 5, which may include solvated metalhydroxide. The heat of reaction in the resultant solution, whichtypically is in the range of about 225° F. to about 300° F., may berecovered and used to warm other segments of the process. It should benoted that some dimethylcarbonate (DMC) will also form in mixing vessel102, but will subsequently decompose. After heat recovery, the methoxide5 is sent to reaction vessel 101 to host the incoming streams of water 4and mostly dry flue gas 1, which is bubbled through the methoxide 5. Theflow rate of the methoxide 5 into reaction vessel 101, as well as theoutflow of water-methanol solution 10 from reaction vessel 101 tocryogenic drying vessel 202 (via first heat exchanger 201) and to thehot distillation column 205, will depend, first, on the flow rate of theflue gas 1 and the CO₂ content of the flue gas. Secondly, the flow rateswill be strictly controlled so as to never allow more than approximately10% water in the reaction vessel because a methoxide medium with alarger moisture content will not as readily precipitate the carbonatesalt.

Methoxide 5 enters reaction vessel 101 into which the flue gas stream 1and water 4 are introduced. Some embodiments may use multiple reactionvessels in series to allow for the constant flow of flue gas. Apreferred reaction vessel has a height of approximately 40 feet and maybe made of stainless steel or appropriately coated carbon steel, or anyother material that can tolerate acids, bases, water and heat withoutcorroding. Reaction vessel 101 is fluidly connected to mixing vessel 102such that the alkali-solvent suspension, here methoxide, enters thereaction vessel through a first input. As discussed in more detailherein, flue gas stream 1 arrives in reaction vessel 101 through asecond input having given up some its heat content in a hot distillationstep associated with the regeneration of the methanol. The chemicalprocess in the reaction vessel can be summarized by the followingequation:

The first step in (1) above is the physical dissolution of carbondioxide gas in the substantially non-aqueous solvent. This dissolutionis reversible, as indicated by the double-headed arrows. The second stepin (1) is the capture of CO₂ by the water or the base to form smallamounts of carbonate in the free form (carbonic acid, H₂CO₃) andcarbonate ions. Ion formation depends on the alkalinity of the solution.The reactions are fast, virtually instantaneous. The carbonate ions areremoved from the vessel as metallic salts (e.g., calcium carbonate orpotassium carbonate) that precipitate to the bottom, thus allowing thereaction to continue. The alkalinity of the solution and the solubilityof the metallic carbonates in the solvent determine the rate ofcarbonate formation and precipitation. Therefore the actual operation ofthe reaction will be optimized by controlling the alkalinity of thesolvent and the temperature, pressure and flow rates of the variousstreams, relative to the solubility of the selected carbonate product.

Preferably, the water produced from the acid-base reaction should notexceed approximately 10% of the volume of the methanol in the reactionvessel. Water control is achieved by constantly drawing offwater-solvent solution 10 from the reaction vessel and replacing it withpure, regenerated methanol. This solvent regeneration process isdiscussed in detail below.

The reaction of alkali 2 and carbonic acid 14 produces a carbonate 6that precipitates to the bottom of reaction vessel 101, where it isremoved by auger 104 or any other device or system that can mechanicallyremove precipitated carbonate. If KH is used as the alkali, some portionof the carbonate 6 will likely stay in solution in the methanol, andwill leave with the water-methanol solution 10 and fall out later duringcryogenic drying. The removed material may undergo drying by recoveredheat from elsewhere in the process, yielding a fine powder or pellets.The carbonate 6 that falls to the bottom of reaction vessel 101 maycarry with it a small amount of methanol, but preferably will not carrywater. The reaction will cause the water-methanol solution product 10 torise upward in reaction vessel 101, while the precipitating carbonate 6will fall toward the bottom.

Thus, the design of the reaction vessel takes advantage of the risingliquid and flue gas streams and the falling carbonate. For example, themethoxide 5 and cool flue gas 1 enter near the bottom of reaction vessel101, while the warmer water-methanol solution 10 is withdrawn near thetop, with the inert gases (N₂, and in some instances O₂) moving on tofurther processing steps in nitrogen liquefaction assembly 300, shown inFIG. 3 and in more detail in FIG. 4. Any methanol (in the form ofwater-methanol solution 10) that leaves reaction vessel 101 with thecarbonate is allowed to evaporate. The dry carbonate would be sent toend-users for use as fertilizer, a lime substitute, in mine reclamation,road fill, or other industrial uses. A substantial percentage of theacidic oxides of nitrogen contained in the flue gas stream will alsoreact with the alkali in the methoxide, yielding various saltscontaining nitrogen, including but not limited to nitrides, thusreducing the emissions from the power plant.

The carbonate 6 produced from the reaction of carbonic acid 14 andalkali 2 depends on the selected alkali. One possibility is calciumcarbonate, which can be used as a substitute for lime in agriculturalfertilizer, or in steel making, oil drilling, diapers, and glass making.Another potential product is magnesium carbonate, which may be used as afertilizer as a substitute for dolomitic limestone, allowing for theavoidance of liming, resulting in the avoidance of CO₂ emissions byreducing the CO₂ emitted during lime production. Potassium carbonate isanother possible product that can be used as a fertilizer and alsoavoids liming. Other potential end products of embodiments of theinvention may include silicon nitride (Si₃N₄), calcium nitride (Ca₃N₂),or magnesium nitride (Mg₃N₂), when metals are burned in pure nitrogen.The separation of argon (as liquid argon) from the liquid nitrogenproduct stream is especially appealing because the nearly 1% argoncontent of the flue gas will yield a high-value liquid argon stream if acold distillation column is included in the LN₂ production loop.

With the CO₂ removed from the flue gas 1 and chemically converted to acarbonate 6, the remaining portion of the flue gas is mostly nitrogen.Stream 8, which contains nitrogen and some methanol, leaves the top ofreaction vessel 101. The hotter the reaction, the more vaporizedmethanol will leave with the N₂ gas. Reaction temperatures of more than150° F. will cause too much methanol to leave the vessel with the N₂.Thus, the heat of reaction needs to be controlled. For example the inletmethoxide stream 5 to reaction vessel 101 may be pre-cooled.Alternatively, reaction vessel 101 may be cooled internally by a heatexchanger suspended near the top of the vessel, for example, using acold N₂ stream 9, to cool the liquid in the reaction vessel to maintainits methanol content in a condensed (liquid) state, allowing theremaining N₂ vapor to move on to nitrogen liquefaction assembly 300 forliquefaction. Preferably, the reaction is allowed to reach near 150° F.,tolerating some methanol boil off, but recovering that methanolimmediately after it leaves reaction vessel 101 in solvent condenser103.

The methods of controlling the temperature in the reaction vessel caninclude cooling the inlet streams (methoxide, water, etc.) and/orcooling the liquids in the reaction vessel by an internal heatexchanger, and/or a combination of those techniques. Those options arenot illustrated in FIG. 1. Those familiar with the engineering of suchheat control systems would select an optimal method. The extent to whichthe reaction vessel needs to be cooler than 150° F. will be determinedby thermodynamic calculations that optimize the rate of the reaction butwithout causing excessive methanol boil off from the reaction vessel.

The stream that leaves solvent condenser 103 is flue gas with mostly N₂7, but it may also include argon, and low amounts of O₂, depending onthe source of the flue gas. Trace amounts of water or CO₂ (parts permillion) would be removed in a molecular sieve 305 (shown in FIG. 4)prior to the liquefaction of the N₂ stream 7 as discussed below. Much ofthe N₂ can be cost-effectively compressed and chilled, and thusliquefied by processes known in the art, to yield liquid nitrogen (LN₂)of a relatively high purity, but at much lower costs than can beproduced at standard air separation plants. This process is performed bynitrogen liquefaction assembly 300, shown in FIG. 3 and FIG. 4.

Turning to FIG. 2, solvent regeneration assembly 200 is shown in moredetail. Solvent regeneration assembly 200 is fluidly connected toreaction vessel 101 and comprises first heat exchanger 201, cryogenicdrying vessel 202 fluidly connected to the first heat exchanger, and hotdistillation vessel 205 fluidly connected to the first heat exchanger.Additional heat exchangers may be used and will be described herein.Water-methanol solution 10 is sent to first heat exchanger 201, where itis deeply chilled by heat exchange with liquid N₂ 9 that has been pumped(by a cryogenic pump, not shown) to a high pressure, e.g., approximately800 psia, or any other pressure suitable for the power enhancementfeatures discussed below. The deeply chilled water-methanol solution 10is then sent to cryogenic drying vessel 202, where the now nearly frozenwater it contains (a “slush” of water with small amounts of methanol)falls to the bottom of the cryogenic drying vessel 202, allowing thatmostly water stream 11 to be drawn off from the bottom 212 of cryogenicdrying vessel 202, and leaving a mostly methanol stream to be drawn offfrom the top 211 of the vessel. If KH is being used as the alkali, someof the carbonate will fall out in the cryogenic drying vessel 202.

In some embodiments, water-methanol stream 10 will carry carbonates insolution with the methanol. Those solids will precipitate toward thebottom 212 of the cryogenic drying vessel 202 and would be removed bymechanical means from the bottom of the vessel, with water-methanolstream 11 removed as mostly water from a higher point on vessel 212.Neither streams 11 nor 12 will carry any solids with them as they moveon in the cycle.

Next, the mostly water stream 11 travels on to the second heat exchanger203, which is preferably an ambient air heat exchanger, for warming.Other sources of heat may include various heat-carrying streams, such asstream 7, in FIG. 1, after that stream leaves solvent condenser 103.That choice would serve to pre-cool the N₂ stream before it arrives atnitrogen liquefaction assembly 300 for liquefaction. From second heatexchanger 203, the mostly water stream 11 enters third heat exchanger204, where it is further warmed by methanol vapor 3 that is driven offfrom the hot distillation vessel 205. For the sake of clarity, thirdheat exchanger 204 is shown directly between second heat exchanger 203and distillation column 205. A fully engineered version of the processwill likely place third heat exchanger 204 above distillation column205, allowing the reflux solvent stream that travels through controlvalve 207 to fall into the column by gravity. Alternatively, a smallpump would move the reflux stream from 204 to 205.

The methanol vapor 3 used in third heat exchanger 204 preferably isapproximately 150° F. and higher, substantially pure methanol vapor.Water may be recovered from hot distillation vessel 205 and used to warmthe N₂ stream as it leaves first heat exchanger 201, on its way to itspower enhancement function in power plant 400, the power cycle whichproduces the flue gas in the first place, and which powers the nitrogenliquefaction assembly 300. Methanol stream 3, which is a vapor at thispoint, is condensed to a liquid by the mostly water stream 11, allowingrecovered methanol 12 to be sent back to mixing vessel 102 for furthermethoxide production. The resulting methoxide suspension may containsome water.

That stream 12, (with very little water content) is removed from the topof cryogenic drying vessel 202, as a dry methanol and returned throughfirst heat exchanger 201 (recovering its coldness) and then joining thereturn stream that exits third heat exchanger 204, with the combinedmostly-methanol stream 12 sent back to mixing vessel 102. The returnflow of stream 12 (mostly dry methanol) travels through first heatexchanger 201, helping the liquid N₂ to cool the water-methanol stream10 from the reaction vessel 101.

The mostly water stream 11 that leaves cryogenic drying vessel 202 andis warmed in second heat exchanger 203 and third heat exchanger 204, isheated in hot distillation vessel 205, driving off its limited contentof methanol vapor and allowing pure water to leave the bottom of the hotdistillation vessel 205. The heat source for this distillation is thehot flue gas 41 which travels through re-boiler 206 at the bottom of hotdistillation vessel 205. The hot flue gas gives up much of its heat inthis step, but still has enough remaining heat that can be recovered foruse elsewhere. Most of the recovered water 4 that leaves hotdistillation vessel 205 is sent back to reaction vessel 101 so that theCO₂ in the flue gas can form carbonic acid 14, as illustrated in FIG. 1.Any extra water that may be produced can be sent through one or morelayers of activated charcoal filtration, after it leaves hotdistillation vessel 205, allowing that water to be potable.Alternatively, excess recovered water may be sent to the steam cycle ofthe power plant as a source of make-up water, replacing water lost inthe steam cycle. Flue gas from natural gas fired power plants will havea higher water content, requiring less of the water 4 recovered from hotdistillation vessel 205 to be returned to reaction vessel 101 to formcarbonic acid with the CO₂ in the flue gas.

Low-pressure methanol vapor 3 leaves the top of hot distillation vessel205 (also known as a distillation column). The heat of that vapor isused to pre-warm the cold (mostly water) stream 11 that is sent to thehot distillation vessel 205. That heat exchange causes the methanolvapor 3 to condense. A portion of the condensed methanol stream is sentback to the top of the hot distillation vessel 205 as a type of refluxstream, which helps vaporize the methanol in the mostly water mixturebelow it. Preferably, the portion of the condensed methanol stream sentback to the top of hot distillation vessel 205 is approximately 10% ofthe stream. Valve 207 is shown on the reflux line, prior to the stream'sentry into the vessel.

The liquid N₂ stream 9 travels through first heat exchanger 201, deeplychilling (to between about −50 and −80° F.) water-methanol stream 10.The flow rate of the liquid N₂ 9, through first heat exchanger 201,controls the exit temperature of the vaporized liquid N₂ (now N₂). In apreferred embodiment, the vaporized N₂ is cold enough to serve as therefrigerant in solvent condenser 103 that condenses the methanolcontained in the mostly-N₂ stream that leaves reaction vessel 101 (asseen on FIG. 1). That side-loop of N₂, having helped condense themethanol in the outflow stream 8 from reaction vessel 101, rejoins thehigh-pressure N₂ stream that leaves first heat exchanger 201, and issent on to do power enhancement work in the basic power productioncycle. Solvent condenser 103 recovers the heat content of theN₂+methanol stream 8 that leaves the warm reaction vessel 101, andtransfers that heat to the cool N₂ side-stream 9 that leaves first heatexchanger 201, and which rejoins the main N₂ stream 7, on its way to thepower cycle. This allows the acid+base reaction in the vessel to occurat the hottest conditions, yielding valuable low-grade heat that istransferred to the N₂ stream 7, shown rejoining the main N₂ stream thatleft heat exchanger 201. The warming of that N₂ stream that is travelingfrom 201 toward subsystem 400 is achieved by the cooling of N₂ stream 7that leaves solvent condenser 103, on its way to liquefaction insubsystem 300.

It should be noted that the distillation of the water-methanol solution10 that is drawn off from reaction vessel 101 can occur in several ways,including by heat (such as from the heat content of the flue gas), byheat augmented by a partial vacuum to draw off the methanol vapor fromthe hot distillation vessel 205, or by vapor recompression methods.However, all those methods would require more heat than is available inthe flue gas. Instead, the present invention “pre-distills” the wetmethanol stream and deeply chills the water-methanol solution 10 suchthat the denser water travels to the bottom of a container and allowsthat saturated methanol stream to be further distilled by any one or acombination of the above methods.

A preferred embodiment shown in FIG. 2 relies on off-peak power storedin the form of liquid N₂ to achieve the distillation (regeneration) ofthe water-methanol solution 10. The cold distillation step yields amostly-water stream, out of which the remaining methanol is distilled byheat. The preferred two-step (cold and hot) regeneration processrequires much less heat to distill the water-methanol solution 10 if theratio of water is very high relative to the ratio of methanol, as is thecase for the arriving mostly water stream 11 that is sent to hotdistillation vessel 205. The net energy required to regenerate themethanol will be less when refrigeration is included in embodiments ofthe invention, because the wider temperature range (between the hot andcold sides of the distillation) allow for a good deal of heat and coldrecovery. Additionally, the production of liquid N₂ will yield a gooddeal of low-cost refrigeration. It should be noted that FIG. 2 does notshow every possible heat recovery step that may optimize the efficiencyof the process and shows only one control valve. Other valves, gauges,sensors, instruments and pumps are not shown.

FIG. 3 shows an embodiment of a carbon capture and sequestration processand system integrating several subsystems, including the inflow andoutflow streams to a power plant, as well as the streams between thesubsystems. These include carbon capture assembly 100, solventregeneration assembly 200, nitrogen liquefaction assembly 300 and thepower production assembly 400. This last part can include coal-fired andbiomass steam cycles, natural gas fueled combined cycles, landfillgas-fired or anaerobic digester-fired plants, and any other hydrocarbonfueled, CO₂-emitting power production systems.

LN₂ production occurs in nitrogen liquefaction assembly 300 with mostlyN₂ as the feed gas. In one example, the LN₂ production stream at a 500MW coal-fired power plant will be approximately 30,000 tons per day.That 30,000 tons per day includes about 0.9% argon, which is alsovaluable, and which is separated from the LN₂ and used to generateincome. In a preferred embodiment, the LN₂ is divided into threeportions. A first portion is sold as a high-value product to off-siteend users, for refrigeration applications and as a product that is usedin oil and gas fields to move such resources to (and up) the wellcasing.

A second portion is used to regenerate the methanol by cryogenic drying,as shown in FIG. 2. That same N₂, after it is vaporized by heatexchange, is sent as a high pressure stream into the steam cycle of apower plant, increasing the mass flow through the steam turbine, or to aseparate hot gas expander which is generator-loaded, thus enhancing thepower output by some 6.5%, without the use of additional fuel. Thehigh-pressure of the N₂ stream is achieved by first pumping the LN₂ topressure, and the heat is absorbed in the high-pressure stream throughthe various heat recovery steps shown in FIG. 2 and discussed herein.

Sources of heat provided by embodiments of the invention for warming thehigh-pressure N₂ vapor include the following: warm water-solventsolution 10 that leaves reaction vessel 101 on its way to regeneration,as shown in FIG. 1, where heat exchange occurs between N₂ stream 9 andwater-solvent solution stream 10 in heat exchanger 201; warm N₂ leavingthe reaction vessel 101, as shown in FIG. 1, where N₂ stream 9 is warmedby the methanol-containing N₂ stream 8 in solvent condenser 103; theremaining heat in the flue gas 1 after it gives up some of its heat inthe hot distillation column 205; heat contained in the recovered water 4from the hot distillation column 205; heat produced by the ionicreaction between the selected alkali 2 and the methanol 12 during themaking of methoxide 5 in mixing vessel 102; the condensation of steam inthe power cycle, normally performed by a cooling tower, which isreplaced by the cold N₂ stream; and in natural gas fired, combined cyclepower plants, the heat absorbed from using cold N₂ as a cooling streamto chill the ambient inlet air to the gas turbine.

A third portion of the daily LN₂ production is stored in one or morecryogenic storage tanks 307, and released during the peak power demandperiod to further enhance the power production cycle. The release ofthat stored energy occurs by first pumping the LN₂ to pressure,preferably using a cryogenic pump, then vaporizing it with waste heatfrom elsewhere in the process, then sending the high-pressure hot N₂stream through a generator-loaded hot-gas expander. That power outputwill increase the peak period power output by another approximately 5%,which, combined with the 6.5% power increase produced during the rest ofthe day, yields a total power boost of about 11% during the peak outputperiod when the power is most valuable. The LN₂ used for that powerenhancement embodiment is preferably made at night using off-peak power,and its storage for later power release constitutes a utility-scalepower storage mode, without batteries, fly wheels or compressed aircavern storage systems.

This storage and release mode, with outflow during peak power demandperiods, constitutes a power storage strategy that converts low-costliquid nitrogen produced as a byproduct of the CO₂ capture process andconverts that recovered nitrogen stream into high-value peak periodpower. The generator-loaded hot gas expander that converts the hot,pressurized nitrogen gas into electric power may be the same expanderthat converts the first portion of nitrogen that was warmed in themethanol regeneration process.

Nitrogen stream 7 is already separated from the air that was initiallyused to combust the fuel used in the power plant 400 (with the O₂content of the air used to combust the fuel), and is also separated fromthe CO₂ contained in the flue gas that resulted from the combustion offuel in air. Any trace amounts of water and CO₂ remaining in thenitrogen stream 8 that leaves reaction vessel 101 can be removed bymolecular sieve 305, preferably containing zeolite. The water and CO₂content of the N₂ stream will be substantially less than that of ambientair, requiring a smaller mole sieve adsorber, or one that is regeneratedless often.

Referring to FIG. 4, nitrogen liquefaction assembly 300 is shown in moredetail. FIG. 4 illustrates N₂ liquefaction using a separate N₂ loop asthe refrigerant, which cools the N₂ stream that leaves carbon captureassembly 100 in a cryogenic heat exchanger 306. N₂ stream 7 is firstcompressed to moderate pressures, e.g., approximately 80 psia, inseveral stages, as represented by multi-stage compressor 302, which isdriven by a motor 301 connected to the compressor by a drive shaft 309.After heat recovery in one or more inter- and after-coolers 303, thecompressed N₂ moves through molecular sieve 305. FIG. 4 shows severallocations where the heat of compression is recovered in heat exchangers(inter- and after-coolers) and is used to provide heat for otherportions of the carbon capture and sequestration process. The compressedN₂ stream is sent to cryogenic heat exchanger 306 where it is chilled toapproximately −280° F. by heat exchange with the refrigerant N₂ streams,shown as 9. The chilling causes the stream to form a mostly liquidphase, which is sent through a pressure letdown/control valve 207between cryogenic heat exchanger 306 and storage apparatus 307,preferably a cryogenic liquid storage tank in which the resultant LN₂ isstored.

The pressure letdown through valve 204 allows more than 90% of thedeeply chilled N₂ 9 to enter the storage tank as a liquid, with lessthan 10% of the stream flashing as a dense, cold (approximately −280°F.) vapor 35. The vapor portion (flash gas) is allowed to leave thestorage tank and is used as small portion of the refrigeration source inthe main heat exchanger that chills the inlet N₂ stream. After giving upits cold to the inlet stream, flash stream 35 is further warmed by heatexchange with other streams (not shown in FIG. 4), sent to molecularsieve 305 as sweep gas to remove the water and CO₂ captured in thesieve, and then vented to the atmosphere through vent 308. That ventstream is benign because it contains mostly N₂ (the main component ofair) with small amounts of water and CO₂.

The main refrigeration loop that liquefies the N₂ stream also uses dryN₂ (or dry air, or any other suitable fluid) as the refrigerant, butwithout mixing the refrigerant stream with the N₂ stream that is to beliquefied. That independent refrigeration loop consists of severalstages of compression and several stages of expansion, (all on a singleshaft 309 or separated on two or more shafts), where an electric motor301 drives the compressor stages 302, and the expander stages 304contribute work that causes the refrigeration, as described below. Thesingle shaft configuration shown for the various stage compressors andexpanders is just one illustrative example of such cryogenicrefrigeration systems. Other layouts, with multiple shafts andvariations on the location of compression and expansion functions can bedesigned by those skilled in the art.

The compressor stages take low-pressure “warmed” refrigerant that leavescryogenic heat exchanger 306 (having deeply chilled the N₂ inlet stream)and bring the refrigerant stream to a high-pressure (e.g., approximately800 psia) in several stages of compression, with the heat of compressionrecovered in inter- and after-coolers 303 for use elsewhere. Thenear-ambient temperature high-pressure refrigerant is then expanded instages in multi-stage expander 304. Those expansions chill therefrigerant to approximately −300° F., but having reduced its pressureto approximately 80 psia. The approximately −300° F. refrigerant coolsan approximately 50° F. N₂ stream to approximately −280° F. in heatexchanger 306. In turn, the inbound N₂ stream 7 warms the refrigerant toapproximately 40° F., requiring it to be re-compressed and cooled byexpansion, in a continuous loop, as described above. The cycle describedhere may have variations, in addition to the possible variationsmentioned above. For example, the inlet N₂ may be compressed to a higherpressure, in various stages, yielding a different proportion of liquidto flash that will enter the LN₂ storage tank, and yielding differentamounts of recoverable heat of compression. An absorption chiller drivenby waste heat of compression and other waste heat sources fromembodiments of the invention may provide pre-cooling of the N₂ stream.

Similar power enhancement is possible at natural gas-fired, combinedcycle power plants, but with the following differences: the N₂ stream isa larger portion of the flue gas stream relative to the CO₂ stream,because natural gas-fired power plants produce less CO₂; and cold N₂ canfirst be sent to cool the inlet air of the gas turbine, and then, oncethe N₂ is warmed up, it can be sent to pick up more heat from waste heatsources in embodiments of the invention, and then to the steam portionof the combined cycle.

The liquefaction cycle requires power input to motors 301 at the N₂stream compressor and at the refrigerant stream compressor, as well asminor amounts of power input for various pumps, instruments and valves.However, that power requirement is substantially offset by the powerenhancement steps described herein, and more than compensated for by thetotal value of the carbonate, the liquid nitrogen and liquid argonsales, the recovered H₂, and the possible recovery of iron oxide fromthe ash and any other byproducts that may be made from the N₂ streamthat is separated from the flue gas. In some embodiments, LN₂liquefaction will likely be done only during off-peak power demandperiods, using lower-value power to produce enough LN₂ for use in themethanol regeneration and power enhancement sequences, and additionalLN₂ for off-site sales. If a cold distillation column is included (notshown in FIG. 4), then liquid argon can be drawn off from the LN₂,yielding another income stream.

Thus, it is seen that carbon capture and sequestration systems andmethods are provided. It should be understood that any of the foregoingconfigurations and specialized components or chemical compounds may beinterchangeably used with any of the systems of the precedingembodiments. Although preferred illustrative embodiments of the presentinvention are described hereinabove, it will be evident to one skilledin the art that various changes and modifications may be made thereinwithout departing from the invention. It is intended in the appendedclaims to cover all such changes and modifications that fall within thetrue spirit and scope of the invention.

What is claimed is:
 1. A method of capturing and sequestering carbondioxide, comprising: mixing a substantially non-aqueous solvent and analkali such that the solvent and alkali form a solvent suspension;mixing water and a flue gas containing carbon dioxide with the solventsuspension in a reaction vessel such that a reaction occurs, thereaction resulting in the rapid formation of a solid dry carbonate,water and heat, the resulting water forming a solution with the solvent,the resulting carbonate being essentially free of organo-metallicproducts and substantially non-aqueous a metallic salt and being one ormore of the carbonates of the group consisting of: ammonium carbonate,lithium carbonate, magnesium carbonate, potassium carbonate, sodiumcarbonate, or calcium carbonate; the resulting carbonate precipitatingout of solution, requiring no further chemical processing steps withoutcarrying water, falling toward the bottom of the reaction vessel, andaccumulating at the bottom of the reaction vessel together with somesubstantially non-aqueous solvent; removing the resulting carbonate fromthe reaction vessel without any water leaving the reaction vessel at thesame location; and evaporating with low-grade heat any remainingnon-aqueous solvent.
 2. The method of claim 1 further comprising thesteps of: continuously removing a portion of the solution of water andsolvent from the reaction vessel; separating the water from the solvent;introducing additional alkali to the separated solvent; re-mixing theseparated solvent with the additional alkali such that the solvent andadditional alkali form a solvent suspension; providing additional fluegas containing carbon dioxide; and returning a portion of the separatedwater to the solvent suspension to continue the reaction.
 3. The methodof claim 2 wherein the precipitated carbonate is mechanically removedfrom the reaction vessel.
 4. The method of claim 1 wherein the solventis an alcohol.
 5. The method of claim 4 wherein the alcohol is methanol.6. The method of claim 5 wherein the alkali reacts with the methanol toform methoxide.
 7. The method of claim 6 further comprising introducingash into the solvent, wherein one or more constituents of the ash arealkalis and the ash contains one or more metal oxides including ironoxide.
 8. The method of claim 2 wherein separating the water from thesolvent includes chilling the solution of water and solvent in acryogenic drying vessel such that the water falls substantially to thebottom of the cryogenic drying vessel and the solvent risessubstantially to the top of the cryogenic drying vessel.
 9. The methodof claim 2 wherein separating the water from the solvent includes thesteps of: applying heat to the solution of water and solvent, using apartial vacuum to draw off vaporous solvent from a hot distillationvessel, and condensing the vaporous solvent.
 10. The method of claim 2wherein carbon dioxide and water react to form carbonic acid.
 11. Themethod of claim 10 further comprising introducing ash into the solvent,one or more constituents of the ash being alkalis and the ash containingone or more metal oxides including iron oxide, wherein the carbonic acidreacts with the alkalis to substantially neutralize the alkalis.
 12. Themethod of claim 11 wherein the reaction of carbonic acid and the alkalisresults in a non-alkaline stream comprising one or more carbonates, sandand iron oxide.
 13. The method of claim 2 wherein the reactiontemperature is less than about 150 degrees Fahrenheit.
 14. A method ofseparating chemical constituents of flue gas, comprising: mixing asubstantially non-aqueous solvent and an alkali such that the solventand alkali form a solvent suspension; introducing water and a flue gascontaining carbon dioxide and nitrogen to the solvent suspension;contacting the alkali in the solvent suspension with the water and thecarbon dioxide in the flue gas in a reaction vessel such that a reactionoccurs, the reaction resulting in the rapid formation of a substantiallynon-aqueous carbonate, water and heat, the carbonate being essentiallyfree of organo-metallic products, and the resulting water forming asolution with the solvent; removing the solution of water and solventfrom the reaction vessel such that the resulting carbonate issubstantially non-aqueous; and substantially liquefying a portion of thenitrogen by compressing and chilling the nitrogen.
 15. A method ofcapturing or sequestering carbon dioxide, comprising: mixing methanoland an alkali such that the methanol and alkali form a solventsuspension; mixing water and a flue gas containing carbon dioxide withthe solvent suspension such that a reaction occurs, the reactionresulting in the rapid formation of a substantially non-aqueous solid,dry carbonate, water and heat, the carbonate being essentially free oforgano-metallic products a metallic salt and being one or more of thecarbonates of the group consisting of: ammonium carbonate, lithiumcarbonate, magnesium carbonate, potassium carbonate, sodium carbonate,or calcium carbonate, the resulting carbonate precipitating out ofsolution without carrying water; and mechanically removing the resultingcarbonate from the reaction vessel without any water leaving thereaction vessel at the same location.
 16. The method of claim 15 whereinthe alkali reacts with the methanol to form methoxide.
 17. The method ofclaim 16 further comprising introducing ash into the solvent, whereinone or more constituents of the ash are alkalis and the ash contains oneor more metal oxides including iron oxide.
 18. The method of claim 15wherein the resulting water forms a solution with the methanol and theresulting carbonate precipitates out of solution, requiring no furtherchemical processing steps, falls toward the bottom of the reactionvessel, and accumulates at the bottom of the reaction vessel togetherwith some methanol.
 19. The method of claim 18 further comprising thesteps of: continuously removing a portion of the solution of water andmethanol from the reaction vessel such that the resulting carbonate issubstantially non-aqueous; separating the water from the solvent;introducing additional alkali to the separated solvent; re-mixing theseparated solvent with the additional alkali such that the methanol andadditional alkali form a solvent suspension; providing additional fluegas containing carbon dioxide; and returning a portion of the separatedwater to the solvent suspension to continue the reaction.
 20. The methodof claim 16 wherein the water content of the methanol does not exceedabout 10% by volume.
 21. A method of capturing and sequestering carbondioxide, comprising: mixing a non-aqueous solvent and an alkali suchthat the solvent and alkali form a solvent suspension; mixing water anda gas containing carbon dioxide with the solvent suspension in areaction vessel such that a reaction occurs, the reaction resulting inthe formation of a solid, dry carbonate, water and heat, the resultingwater forming a solution with the solvent, the resulting carbonate beinga metallic salt; the resulting carbonate precipitating out of solutionwithout carrying water, falling toward the bottom of the reactionvessel, accumulating at the bottom of the reaction vessel together withsome solvent; and mechanically removing the resulting carbonate from thereaction vessel without any water leaving the reaction vessel at thesame location.